Molten salt electrolyzer for reducing metal, method for electrolyzing the same, and process for producing refractory metal with use of reducing metal

ABSTRACT

A molten salt electrolyzer for reducing metal comprises an electrolytic cell filled with a molten salt composed of a reducing metal chloride, an anode immersed in the molten salt of the electrolytic cell and surrounded by a first wall at the periphery thereof, and a cathode immersed in the molten salt of the electrolytic cell and surrounded by a second wall at the periphery thereof.

TECHNICAL FIELD

The present invention relates to an electrolyzer for production of a reducing metal by molten salt electrolysis, and the present invention also relates to a method for electrolyzing the reducing metal.

BACKGROUND ART

In general, titanium metal is often used for aircraft materials and aircraft parts. Recently, titanium metal is more widely used, and it is now used for architectural materials, roads, sports products, etc. Conventionally, titanium metal is produced by the Kroll method in which titanium tetrachloride is reduced with molten magnesium and sponge titanium is obtained, and various improvements have been made to reduce the production cost. However, since the Kroll method is a batch process in which a series of operations is discontinuously repeated, it is difficult to further improve production efficiency.

In view of the circumstances described above, techniques have been suggested. In one of the techniques, titanium oxides are reduced with calcium in a molten salt so as to produce titanium metal (for example, see WO99/064638 and Japanese Unexamined Patent Application Publication No. 2003-129268). In another technique, called the EMR method, a reductant containing a metal such as calcium or an alloy thereof is produced, and a titanium compound is reduced with electrons released from the reductant so as to yield titanium metal (for example, see Japanese Unexamined Patent Application Publication No. 2003-306725).

In the above methods, titanium metal is collected from a reaction system after electrolytic reaction. Then, calcium oxide, which is produced as a by-product, is dissolved in calcium chloride and is electrolyzed in a molten salt so as to collect calcium and reuse the calcium in a production process for titanium metal.

The temperature of an electrolytic bath during the above process is maintained at the melting point of calcium or higher, whereby calcium produced by electrolytic reaction exists in a liquid state. However, molten calcium easily dissolves and disperses into calcium chloride because molten calcium has a high solubility with respect to calcium chloride, and the collection efficiency of calcium may be decreased.

On the other hand, U.S. Pat. No. 3,226,311 discloses a technique in which a complex molten salt having a lower melting point than that of calcium is used, and solid calcium is precipitated on a cathode. In this technique, however, additional complex molten salt is required, and production costs are not reduced.

When calcium produced by molten salt electrolysis comes into contact with chlorine gas, which is produced by a side reaction of the electrolytic reaction, a back reaction occurs, and the calcium returns to the former state and is transformed into calcium chloride. As a result, the back reaction results in a decrease in current efficiency.

As mentioned above, in conventional methods, it is difficult to efficiently collect active metal such as calcium, and production costs will be high even if an active metal can be efficiently collected.

DISCLOSURE OF THE INVENTION

In view of the above circumstances, an object of the present invention is to provide a device for producing a reducing metal which may be used for reducing oxides or chlorides of, for example, titanium, and to provide a method therefor. Specifically, the present invention provides a molten salt electrolyzer for efficiently producing a reducing metal by molten salt electrolysis, a method for electrolyzing the same, and process for producing a refractory metal with the use of the reducing metal.

The inventors have performed research in view of the above circumstances, and they found that reducing metal could be produced while maintaining high current efficiency by disposing walls so as to surround an anode and a cathode, respectively, which form a molten salt electrolytic cell. Thus, the inventors have completed the invention.

That is, the present invention provides a molten salt electrolyzer for reducing metal, the molten salt electrolyzer comprises an electrolytic cell filled with a molten salt composed of a reducing metal chloride, and an anode and a cathode are immersed in the molten salt of the electrolytic cell. A first wall surrounding the periphery of the anode and a second wall surrounding the periphery of the cathode are disposed in the electrolytic cell.

According to the above structure, the anode and the cathode are surrounded by a wall, respectively, thereby preventing a back reaction that occurred by the dispersing of chlorine gas produced and an active metal in the electrolytic cell. Moreover, reducing metal is piled up in the inside of the second wall at a good yield ratio, and it can be efficiently collected, and as a result, molten salt electrolysis can be performed at high efficiency.

In the present invention, each of the first wall and the second wall preferably has an opening so that a molten salt can communicate each other therethrough. According to this structure, the walls prevent dispersion of chlorine gas and a reducing metal while molten salts can communicate each other through the opening, whereby efficiency of molten salt electrolytic reaction may not be decreased.

The first wall preferably extends to the bottom of the electrolytic cell and preferably comprises a porous body. According to this structure, the chlorine gas that is produced forms bubbles, and they rise to the surface of the electrolytic bath without communicating through the porous body, whereas the reaction proceeds by contacting of chloride ions and the anode since the electrolytic bath can communicate through the porous body.

The first wall is preferably made of a metal oxide, a metal nitride, or a metal carbide, and the second wall is preferably made of a metal, a metal nitride, or a metal carbide. According to this structure, chlorine gas will not corrode the first wall, and corrosion of the second wall caused by produced reducing metal is effectively prevented.

In a method for producing a reducing metal relating to the present invention, the above-mentioned molten salt electrolyzer is used. A reducing metal with high purity can be efficiently produced while high current efficiency is maintained, by using the molten salt electrolyzer.

In the present invention, a refractory metal chloride selected from the group consisting of titanium, zirconium, tantalum, and niobium is reduced with the reducing metal produced by the above method.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic section view showing a first and a second embodiments of a molten salt electrolyzer of the present invention.

FIG. 2 is a schematic section view showing a third embodiment of a molten salt electrolyzer of the present invention.

FIG. 3 is a schematic section view showing a fourth embodiment of a molten salt electrolyzer of the present invention.

FIG. 4 is a schematic section view showing a fifth embodiment of a molten salt electrolyzer of the present invention.

EXPLANATION OF REFERENCE NUMERALS

1 a to 1 d denote a molten salt electrolyzer, 11 denotes an electrolytic cell, 12 denotes an electrolytic bath, 21 denotes an anode, 22 denotes a cathode, 31 denotes a first wall, 32 denotes a second wall, 33 denotes openings of a second wall, 34 denotes a wall of a cell, 35 denotes an opening of a wall of a cell, 36 denotes openings of a first wall, 37 denotes a wall, 37 a denotes a ceramic layer, 37 b denotes a metal layer, 41 denotes chlorine gas, 42 denotes a condensed layer of calcium, 51 denotes a nozzle for feeding an electrolytic bath, 52 denotes a nozzle for extracting chlorine gas, 53 denotes a nozzle for extracting a condensed layer of calcium, and 54 denotes a nozzle for extracting an electrolytic bath.

BEST MODE FOR CARRYING OUT THE INVENTION

Preferred embodiments of the present invention will be described with reference to the figures hereinafter. FIG. 1 shows a structural example of a preferred molten salt electrolyzer for practicing the present invention. In the embodiment, a combination of an electrolytic bath and a reducing metal that can be produced may be appropriately selected. As an example, a case (first embodiment) of collecting calcium in a solid state will be described. In this case, an electrolytic bath consists of calcium chloride, the temperature thereof is not more than the melting point of calcium, and calcium is produced as a reducing metal.

FIG. 1 shows a molten salt electrolyzer 1 a equipped with an electrolytic cell 11 which is filled with an electrolytic bath 12 containing a molten calcium chloride and has an anode 21 and a cathode 22 arranged in immersed form therein. A first wall 31 surrounding the periphery of the anode 21, and a second wall 32 surrounding the periphery of the cathode 22, are immersed and disposed in the electrolytic cell 11. The first wall 31 consists of a porous body so that the electrolytic bath 12 can migrate between the outside and the inside of the first wall 31 surrounding the anode 21 by penetrating diffusion. On the other hand, the second wall 32 is made of a dense material so that the electrolytic bath 12 cannot penetrate and diffuse, and it is provided with openings 33 of the second wall in proximity of the level below the cathode 22 so that the inside of the second wall 32 is communicated with the outside thereof.

An operation of the electrolyzer will be described. First, the molten salt electrolytic cell 11 is heated to the temperature of the melting point of calcium chloride or higher by a heating device (not shown in the figure) so as to melt the electrolytic bath 12. Then, a predetermined direct-current voltage is applied between the anode 21 and the cathode 22 so as to start a molten salt electrolysis of the electrolytic bath 12. Chloride ions in the electrolytic bath 12 reach the anode 21 by passing through the first wall 31 and supply electrons to the electrode, thereby producing chlorine gas 41. The produced chlorine gas 41 is collected and is transferred to another process (not shown in the figure), such as a chlorination process, and it is separately reused. On the other hand, calcium ions included in the electrolytic bath 12 reach the cathode 22 by passing through the openings 33 of the second wall and receive electrons, thereby producing calcium. The electrolytic bath 12 is maintained at a temperature that is not higher than the melting point of calcium, whereby calcium is precipitated in a solid state and rises to the surface of the electrolytic bath. Then, the calcium is collected and is transferred to a reduction process of titanium (not shown in the figure) so as to be used.

According to the molten salt electrolyzer 1 a having the above structure, bubbles of chlorine gas 41 produced at the anode 21 do not diffuse to the cathode 22 because of the first wall 31, and most of them rise to the surface of the electrolytic bath. The solid calcium produced at the cathode 22 does not diffuse to the anode 21 because of the second wall 32, while a part of it dissolves into the electrolytic bath 12. Thus, the produced chlorine gas and the calcium do not diffuse and come into contact with each other because of the walls surrounding the electrodes, whereby a back reaction does not occur. Therefore, the efficiency of molten salt electrolysis is improved.

As a second embodiment, in the electrolyzer of FIG. 1, the electrolytic bath may be maintained at a temperature of the melting point of calcium or higher so as to collect calcium in a molten state. In this case, just after calcium is produced by electrolysis, it dissolves into the electrolytic bath in the inside of the second wall 32 and forms a condensed layer 42 of calcium including calcium chloride. The condensed layer 42 of calcium is appropriately extracted and is transferred to a reduction process of titanium (not shown in the figure) so as to be used.

In the molten salt electrolyzer 1 of the second embodiment having the above structure, as in the case of the first embodiment, bubbles of chlorine gas 41 produced at the anode 21 do not diffuse to the cathode 22 because of the first wall 31, and most of them rise to the surface of the electrolytic bath. The calcium produced at the cathode 22 cannot diffuse to the anode 21 because of the second wall 32. As a result, no back reaction of chlorine gas and calcium occurs, whereby the efficiency of molten salt electrolysis is improved.

Next, components of the molten salt electrolyzer of the present invention will be described in detail.

Electrolytic Cell

The electrolytic cell 11 is preferably made of a material that can resist high temperatures during operation and may not react with a molten calcium chloride and a molten calcium. Specifically, the electrolytic cell 11 is preferably made of titanium, tantalum, or niobium.

Electrolytic Bath

The temperature of the electrolytic bath 12 differs according to cases in which calcium in a molten state is collected and calcium in a solid state is collected. When calcium in a molten state is collected, molten salt electrolysis is preferably performed in a temperature range that is higher than the melting point of calcium, and specifically, it is preferable that the temperature range be higher than the melting point of calcium by 5° C. to 50° C. In this case, since the melting point of calcium is 845° C., the temperature of the electrolytic bath is set to be 850° C. to 895° C. In addition, it is more preferable that the temperature range be higher than the melting point of calcium by 5° C. to 20° C. in view of the cost of heating and evaporation of calcium chloride. Since the melting point of calcium chloride is 780° C. and is lower than that of calcium, calcium chloride is maintained in a molten state in the condition that calcium is maintained in a molten state.

On the other hand, when calcium in a solid state is collected, the electrolytic bath is preferably maintained in a temperature range that is not less than the melting point of calcium chloride and is not more than the melting point of calcium. Specifically, the temperature range is preferably not more than 845° C., which is the melting point of calcium, and is preferably not less than the melting point of calcium chloride. In this case, since calcium produced at the cathode easily dissolves into calcium chloride and diffuses in the electrolytic bath, electrolysis is preferably performed at a temperature that is slightly higher than the melting point of calcium chloride. Specifically, electrolysis is preferably performed at a temperature range from 785° C. to 800° C.

The electrolytic bath may consist of not only a single bath containing calcium chloride, but also a mixed salt of a potassium chloride and a calcium fluoride, so as to lower the melting point of the electrolytic bath. As a result, the electrolysis temperature can be lowered compared to a case in which the electrolytic bath is a single bath consisting of calcium chloride, whereby solid calcium is easily precipitated. For example, the addition of only approximately 5 to 20 mol % of potassium chloride or calcium fluoride to calcium chloride is effective.

Anode

The anode is immersed in the molten salt of the electrolytic bath, and chlorine gas is produced thereat by electrolyzing calcium chloride contained in the electrolytic bath. Since the temperature of the electrolytic bath is high and is around 800° C., the anode is exposed to high-temperature chlorine gas and calcium chloride. Therefore, the anode is preferably made of a material that can resist high-temperature chlorine gas and molten salt, and graphite is preferable for industrial purpose. Since graphite is relatively inexpensive and is easily processed, and since it is superior in corrosion resistance with respect to high-temperature molten salt and chlorine gas, it is suitable for an anode of the present invention.

Cathode

The cathode is immersed in the molten salt of the electrolytic bath, and calcium is precipitated thereat by electrolyzing calcium chloride. Since calcium is reducible, the cathode can be made of any material that can resist molten calcium chloride and is superior in electric conduction property, and it may be made of a carbon steel or a stainless steel, for example. The end of the cathode is preferably processed in advance so as to have a surface that is as rough as possible. Specifically, the cathode is preferably sandblasted. In addition, the cathode is preferably threaded at the surface in advance. Such surface treatments can facilitate the precipitation of calcium at the cathode.

First Wall

The first wall is provided in order to prevent a back reaction in which chlorine gas produced at the anode is reduced to calcium chloride by reacting with calcium produced at the cathode, and it is preferably disposed by surrounding the periphery of the anode. In this case, electrolytic reaction cannot continue if the electrolytic bath at the outside of the first wall cannot migrate to the inside thereof. Therefore, in a case in which a first wall without an opening, as shown in FIG. 1, is mounted, the first wall is preferably made of a porous body in which chlorine gas does not diffuse but the electrolytic bath can communicate each other through.

The material for the porous body is preferably selected from the group consisting of a metal oxide, a metal nitride, and a metal carbide that have corrosion resistance with respect to chlorine gas and the molten salt. The porous body is preferably made of a metal oxide that is selected from the group consisting of alumina, silica, zirconia, magnesia, and a compound material of these ceramics. The porous body is preferably made of a metal nitride which is selected from the group consisting of silicon nitride, boron nitride, titanium nitride, zirconium nitride, and tantalum nitride. Moreover, the porous body is preferably made of a metal carbide that is selected from the group consisting of silicon carbide, boron carbide, titanium carbide, zirconium carbide, and tantalum carbide. The porosity of the porous body is preferably in a range from 5 to 30%. Using a porous body having such a porosity prevents the diffusion of chlorine gas produced at the anode and facilitates the migration of the electrolytic bath.

Second Wall

The second wall is preferably disposed so as to surround the periphery of the cathode, thereby effectively preventing the diffusion of calcium produced at the cathode. As in the case of the first wall, the second wall may be made of a porous body. However, calcium floats and remains in the electrolytic bath, whereas chlorine gas forms bubbles, rises into the atmosphere, and is removed from the system, whereby the calcium may penetrate into the second wall and may diffuse to the anode. Therefore, the second wall is preferably made of a material that is as dense as possible. In addition, because the second wall made of ceramics is often reductively corroded by molten calcium, it is preferably made of a metal, a metal nitride, or a metal carbide. The metal is preferably selected from the group consisting of stainless steel, titanium, niobium, and tantalum that have corrosion resistance, and the metal nitride is preferably selected from the group consisting of silicon nitride, boron nitride, titanium nitride, zirconium nitride, and tantalum nitride. The metal carbide is preferably selected from the group consisting of silicon carbide, boron carbide, titanium carbide, zirconium carbide, and tantalum carbide.

It is preferable that the second wall be dense as possible in order to prevent the diffusion of calcium produced at the cathode. However, the second wall is preferably provided with an opening so as to facilitate migration of the electrolytic bath. The opening of the second wall is preferably arranged at a level lower than the end of the cathode by 10 mm or more. If the opening is arranged at a level which is not lower than the lower end of the cathode by 10 mm, calcium, which diffuses from the surface of the bath to the bottom, undesirably disperses into the electrolytic bath at the outside of the second wall by passing through the opening. When the second wall is of a cylindrical shape, a plurality of openings is preferably provided.

Calcium is produced by electrolytic reaction, and a condensed layer made of the calcium alone or the calcium dissolved in calcium chloride is formed in the electrolytic bath surrounded by the second wall. For example, the condensed layer of calcium may be used as a reductant for directly reducing titanium oxide and titanium chloride by immediately extracting from the electrolytic cell to another vessel.

FIG. 2 shows an electrolyzer 1 b of another preferred embodiment (third embodiment) relating to the present invention. In the embodiment, another wall 34 of cell is provided between the first wall 31 and the second wall 32. The wall 34 of cell is immersed and is disposed in the electrolytic cell 12, thereby effectively preventing a phenomenon in which calcium that has flowed out from the second wall 32 diffuses and migrates to the anode. Moreover, it also effectively prevents a phenomenon in which chlorine gas that has flowed out from the first wall 31 diffuses to the cathode.

The wall 34 of cell that is immersed and disposed in the electrolytic bath 12 is preferably provided with an opening 35 of the wall of the cell so that the electrolytic bath 12 can migrate. Providing such an opening facilitates the migration of the bath between the anode and the cathode.

FIG. 3 shows an electrolyzer 1 c of the preferred embodiment (the fourth embodiment) when the present invention is applied to actual equipment. In the embodiment, some nozzles for feeding or extracting are added and are arranged on the basis of the third embodiment. Specifically, a nozzle 51 for feeding electrolytic bath 12, a nozzle 53 for extracting condensed layer 42 of calcium so as to send the condensed layer 42 of calcium to a next process, and a nozzle 54 for extracting electrolytic bath are added and disposed. The first wall 31 of the first and the second embodiments is modified in a shape for covering the entire anode 21, and it is disposed as a nozzle 52 for extracting chlorine gas so that chlorine gas 41 produced at the anode 31 is extracted to the outside of the system without a leak.

The first wall 31 (52) is provided with openings 36 of first wall at the lower end so that the electrolytic bath 12 can migrate. In the embodiment designed for actual equipment, the first wall 31 is preferably made of a metal oxide, a metal nitride, or a metal carbide so as to avoid penetrative diffusion of the electrolytic bath 12, and it is more preferable that a metal nitride be used therefor.

The openings 36, 35, and 33 are provided at the first wall 31, the wall 34 of the cell, and the second wall 32, respectively, and their positions are preferably not at the same level but at different level. Such an arrangement of the openings effectively prevents a back reaction, which occurs by a reaction of chlorine gas produced at the anode and calcium, even if calcium or a condensed layer 42 of calcium produced at the cathode 22 reaches the opening, because the electrolytic bath 12 flows up and down to the anode 21.

FIG. 4 shows an electrolyzer 1 d of another preferred embodiment (the fifth embodiment) relating to the present invention. This embodiment comprises an electrolytic cell 11, an anode 21, a cathode 22, a wall 37, and an electrolytic bath 12, and it has the same basic features as the above first to the fourth embodiments. In this case, in the embodiment, the first wall 31 and the second wall 32 of the above first to the fourth embodiments are connected together and form a sheet as wall 37. The wall 37 is made of a cladding material comprising a ceramic layer 37 a at the anode side and a metal layer 37 b at the cathode side, which is different from the case of the above first to the fourth embodiments.

The ceramic layer 37 a and the metal layer 37 b forming the wall 37 are preferably made of the same material as the material for the above-mentioned first wall 31 and the second wall 32, respectively. In addition, the ceramic layer 37 a is preferably made of a metal oxide. When the ceramic layer 37 a is made of a metal nitride, the metal layer 37 b is not required, and the wall 37 can be formed by only the ceramic layer 37 a.

In the embodiment, there is one wall 37, which is immersed in the electrolytic bath, whereby the electric resistance between the anode 21 and the cathode 22 is decreased compared to the cases of the above-mentioned embodiments. As a result, the molten salt electrolysis requires little electric power, and the electric power consumption rate of calcium can be decreased.

Calcium can be effectively produced by molten salt electrolysis of calcium chloride using each of the above-mentioned embodiments of the present invention, whereas it has been difficult to do so by conventional techniques.

Moreover, titanium can be produced by using calcium, which is produced by the above method, as a reductant for titanium tetrachloride. Zirconium, tantalum, and niobium can be efficiently produced by using the calcium as a reductant for a chloride of zirconium, tantalum, or niobium instead of the titanium tetrachloride.

EXAMPLES

The present invention will be described in more detail with reference to the following examples.

First Example

An electrolyzer shown in FIG. 1 comprising the following device configuration was used and was filled with calcium chloride (100%) as an electrolytic bath, and the electrolytic bath was maintained at 880° C.

Device Configuration

Electrolytic cell: titanium crucible

Anode: graphite

Cathode: carbon steel

First wall: silicon nitride tube

Second wall: titanium tube

Results

Molten salt electrolysis of calcium chloride was performed under the above conditions, and a black solution that seemed to be calcium was produced in the proximity of the surface of the bath at the cathode. Current efficiency was obtained by comparing an amount of calcium collected by molten salt electrolysis and a theoretical amount of precipitated metal that was calculated from the amount of applied electric current. The current efficiency was 75%. The collected calcium was analyzed, and the purity thereof was 80%.

Second Example

Molten salt electrolysis was performed under the same condition as that of the first example, except that the temperature of the electrolytic bath was maintained at 800° C. so as to precipitate calcium in a solid state at the cathode. As a result, solid calcium was precipitated at the surface of the cathode. The current efficiency was obtained by comparing the amount of the solid calcium and a theoretical amount of precipitated metal that was calculated from an amount of applied electric current. The current efficiency was 85%. The precipitated calcium was analyzed, and the purity thereof was 90%.

Third Example

An apparatus shown in FIG. 3 was used, and molten salt electrolysis of calcium chloride was performed for 5 hours under the same condition as that of the first example, except that the temperature of electrolytic bath was 800° C. The current efficiency was obtained by comparing the amount of the calcium collected by molten salt electrolysis and a theoretical amount of precipitated metal that was calculated from the amount of applied electric current. The current efficiency was 80%. The collected calcium was analyzed, and the purity thereof was 90%.

First Comparative Example

Molten salt electrolysis of calcium chloride was performed under the same condition as that of the first example, except that the second wall 32 was not used. The current efficiency was obtained by comparing the amount of the calcium collected by molten salt electrolysis and a theoretical amount of precipitated metal that was calculated from the amount of applied electric current. The current efficiency was 25%.

Second Comparative Example

Molten salt electrolysis of calcium chloride was performed under the same conditions as that of the first example, except that the first wall 31 was not used. The current efficiency was obtained by comparing the amount of the calcium collected by molten salt electrolysis and a theoretical amount of precipitated metal that was calculated from the amount of applied electric current. The current efficiency was 40%.

INDUSTRIAL APPLICABILITY

According to the above-described examples and comparative examples, calcium can be efficiently produced and be collected by molten salt electrolysis of metal chloride, specifically, calcium chloride using the present invention. Moreover, a refractory metal chloride can be reduced by using the produced calcium. 

1. A molten salt electrolyzer for reducing metal, comprising: an electrolytic cell filled with a molten salt composed of a reducing metal chloride; an anode and a cathode immersed in the molten salt of the electrolytic cell; a first wall surrounding the periphery of the anode; and a second wall surrounding the periphery of the cathode, wherein the first wall extends to the bottom of the electrolytic cell and comprises a porous body.
 2. The molten salt electrolyzer for reducing metal according to claim 1, wherein the first wall and the second wall have an opening through which the molten salt can communicate each other.
 3. (canceled)
 4. The molten salt electrolyzer for reducing metal according to claim 1, wherein the anode is made of graphite.
 5. The molten salt electrolyzer for reducing metal according to claim 1, wherein the cathode is made of one of two materials among a carbon steel and a stainless steel.
 6. The molten salt electrolyzer for reducing metal according to claim 1, wherein the first wall is made of one of a metal oxide, a metal nitride, and a metal carbide.
 7. The molten salt electrolyzer for reducing metal according to claim 1, wherein the second wall is made of one of a metal, a metal nitride, and a metal carbide.
 8. The molten salt electrolyzer for reducing metal according to claim 6, wherein the metal oxide comprises at least one selected from the group consisting of alumina, silica, zirconia, and magnesia.
 9. The molten salt electrolyzer for reducing metal according to claim 6, wherein the metal nitride is selected from the group consisting of silicon nitride, boron nitride, titanium nitride, zirconium nitride, and tantalum nitride.
 10. The molten salt electrolyzer for reducing metal according claim 6, wherein the metal carbide is selected from the group consisting of silicon carbide, boron carbide, titanium carbide, zirconium carbide, and tantalum carbide.
 11. The molten salt electrolyzer for reducing metal according to claim 7, wherein the metal is selected from the group consisting of titanium, niobium, tantalum, and stainless steel.
 12. The molten salt electrolyzer for reducing metal according to claim 1, wherein the reducing metal is calcium.
 13. The molten salt electrolyzer for reducing metal according to claim 1, wherein the molten salt is selected from the group consisting of calcium chloride, a mixed salt of calcium chloride and potassium chloride, and a mixed salt of calcium chloride and calcium fluoride.
 14. A process for producing a reducing metal by molten salt electrolysis using the molten salt electrolyzer for reducing metal according to claim
 1. 15. The process for producing a reducing metal by molten salt electrolysis according to claim 14, wherein a temperature of the molten salt is maintained in a temperature region that is higher than the melting point thereof by 5 to 50° C.
 16. The process for producing a reducing metal by molten salt electrolysis according to claim 14, wherein the reducing metal is calcium.
 17. A process for producing a refractory metal by reducing the refractory metal chlorides, which is selected from the group consisting of titanium, zirconium, tantalum, and niobium, with the reducing metal produced by the process according to claim
 14. 18. The molten salt electrolyzer for reducing metal according to claim 7, wherein the metal nitride is selected from the group consisting of silicon nitride, boron nitride, titanium nitride, zirconium nitride, and tantalum nitride.
 19. The molten salt electrolyzer for reducing metal according to claim 7, wherein the metal carbide is selected from the group consisting of silicon carbide, boron carbide, titanium carbide, zirconium carbide, and tantalum carbide. 